tests: add a comment that we're purposely testing py2 extension attributes
Avoid someone unknowingly removing test coverage. There are tests for a
properly byteified testedwith a few lines down. I don't see similar for
buglink, but it's a trivial conversion to bytes, so I'm not concerned about
testing the expected/wanted extension state.
Differential Revision: https://phab.mercurial-scm.org/D9434